1. Field of the Invention
This invention relates to a method of and an apparatus for reproducing a negative or a positive image corresponding to an original image as desired, and more particularly to an image forming apparatus which, by simple construction, enables a positive electrostatic image and a negative electrostatic image corresponding to an optical image of an original to be selectively formed as a high contrast image on a photosensitive medium having an insulating layer on the surface thereof.
2. Description of the Prior Art
The need for selective reproduction of a negative and a positive image of an original image has been very great in machines such as reader-printers or the like. This is because, in reader-printers wherein the images recorded as negative or positive images on rolled film, fiche or the like are copied and reproduced, it would be highly convenient in using the copies if the negative images could be reproduced as positive images and the positive images could be reproduced as they were.
Nevertheless, the most prevailing print system for reader-printers which require compactness has been based on the silver salt photography method. However, the use of silver salt print paper could not provide positive print of an original image which may be negative or positive. Moreover, the quality of the printed image has been susceptible to variation with time, as a result of which the contrast of the printed image has been reduced or the image has been discolored. A further disadvantage is that the paper in use has been expensive special paper.
Further print systems heretofore proposed include the Fax system which uses photsensitive paper based on electrophotography, and the transfer system whereby an image formed on a photosensitive medium is transferred to ordinary paper.
In the Fax system, problems have been encountered in that the use of the special paper adds to the running costs and that relatively great thickness of the paper makes writing thereon difficult. Further, in order to selectively reproduce negative and positive images with this system, use must be made of Fax paper which has a bipolarity characteristic; that is, it has a photosensitive layer capable of retaining both positive and negative charges in the dark, wherein the charging polarity must be changed in accordance with the selection of the negative or the positive polarity for the image to be reproduced. This corresponds to the polarity of the toner in the developer and for example, when the toner in the developer is of negative polarity, the charging polarity is positive in case of the positive-positive reproduction, while the charging polarity is negative in case of negative-positive reproduction. However, the characteristic of such paper of opposite polarities is not sufficient at present and can only provide images of poor quality. On the other hand, the transfer system is very satisfactory in terms of image quality, as compared with the foregoing two systems, and does not suffer from any image degeneration due to aging. It also ensures low running costs. According to the transfer system, however, practical machines are designed exclusively for the reproduction of a positive image of an original or for the reproduction of a negative image of an original and, where image inversion is required, inversion development is effected by using a developer of the opposite polarity. As in the Fax system, change-over of the charging polarity is conceivable but in such case, the photosensitive medium is required to have bipolarity characteristic as in the previously described case. Thus, improvement of the characteristic of the photosensitive medium is imperative to provide good images of either polarity as is also the case with the Fax system.
Further, a high voltage source for applying voltages of positive and negative polarities to chargers will be needed in any Fax system and transfer system and this will unavoidably lead to an increase in the number of parts and higher costs, and the addition of such high voltage source is also undesirable. When the above-mentioned inversion development is to be carried out, replacement of the developer will be involved and this will necessitate the provision of a plurality of developing devices. In view of these considerations, the present invention provides a method and an apparatus which will achieve the objectives hereinafter described.